Microporous polyolefin membranes can be used as battery separators in, for example, primary and secondary lithium batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries, etc. When microporous polyolefin membranes are used for battery separators, particularly lithium ion battery separators, the membranes' performance significantly affects the properties, productivity and safety of the batteries. Accordingly, the microporous polyolefin membrane should have suitable mechanical properties, heat resistance, permeability, dimensional stability, shut down properties, melt down properties, etc. As is known, it is desirable for the batteries to have a relatively low shut down temperature and a relatively high melt down temperature for improved battery safety properties, particularly for batteries that are exposed to high temperatures during manufacturing, charging, re-charging, use, and/or storage. Improving separator permeability generally leads to an improvement in the battery's storage capacity. High shut down speed is desired for improved battery safety, particularly when the battery is operated under overcharge conditions. Improving pin puncture strength is desired because roughness of the battery's electrode can puncture the separator during manufacturing leading to a short circuit. Improved thickness uniformity is desired because thickness variations lead to manufacturing difficulties when winding the film on a core. Thickness variations can also lead to non-isotropic temperature variations in the battery, which can lead to battery hot-spots (regions of higher temperature) where the separator is relatively thin.
In general, microporous membranes containing only polyethylene (i.e., the membrane consists of, or consists essentially of, polyethylene) have low melt down temperatures, while microporous membranes containing only polypropylene have high shut down temperatures. Accordingly, microporous membranes comprising polyethylene and polypropylene as main components have been proposed as improved battery separators. It is therefore desired to provide microporous membranes formed from polyethylene resins and polypropylene resins, and multi-layer microporous membranes comprising polyethylene and polypropylene.
JP7-216118A, for example, discloses a battery separator having a suitable shut down temperature and mechanical strength. The patent publication discloses a battery separator comprising a multi-layer porous film having two microporous layers. Both layers can contain polyethylene and polypropylene, but in different relative amounts. For example, the percentage of the polyethylene is 0 to 20 wt. % in the first microporous layer, and 21 to 60 wt. % in the second microporous layer, based on the combined weight of the polyethylene and polypropylene. The total amount of polyethylene in both microporous layers is 2 to 40 wt. %, based on the weight of the multi-layer microporous membrane.
JP10-195215A discloses a relatively thin battery separator having acceptable shut down and pin-pulling characteristics. The term “pin pulling” refers to the relative ease of pulling a metal pin from a laminate of a separator, a cathode sheet and an anode sheet, which is wound around the pin, to form a toroidal laminate. The multi-layer porous film contains polyethylene and polypropylene, but in different relative amounts. The percentage of polyethylene is 0 to 20 wt. % in the inner layer and 61 to 100 wt. % in the outer layer, based on the total weight of the polyethylene and polypropylene.
JP10-279718A discloses a separator designed to prevent unacceptably large temperature increases in a lithium battery when the battery is overcharged. The separator is formed from a multi-layer porous film made of polyethylene and polypropylene, with different relative amounts of polyethylene and polypropylene in each layer. The film has a polyethylene-poor layer whose polyethylene content is 0 to 20 wt. %, based on the weight of the polyethylene-poor layer. The second layer is a polyethylene-rich layer which contains 0.5 wt. % or more of polyethylene having a melt index of 3 or more and a total polyethylene content of 61 to 100 wt. %, based on the weight of the polyethylene-rich layer.
It is desirable to further improve the permeability, pin puncture strength, and shutdown speed of microporous polyolefin membranes. Moreover, it is desirable to further improve the thickness uniformity of microporous polyolefin membranes in order to reduce the likelihood of short-circuiting when used as battery separators. Particularly, it is desirable to provide a microporous membrane having excellent electrochemical stability and low heat shrinkage, as well as high permeability and heat resistance, when used as a battery separator.